1. Field of the Invention
The present invention relates to a syncword detecting circuit and a baseband signal receiving circuit. More specifically, the present invention relates to a syncword detecting circuit and a baseband signal receiving circuit that is applicable to, for example, a receiver for a radio signal, which complies with a bluetooth system corresponding to a kind of short distance radio link.
2. Background Information
A conventional bluetooth receiver of a bluetooth system corresponding to a radio link using radio signals lying in a 2.4 GHz radio frequency band has an RFIC (Radio Frequency Integrated Circuit) transceiver for extracting a baseband signal having a 1 MHz symbol transmission rate from the abovementioned 2.4 GHz radio frequency band signals, and a baseband receiving circuit for packet assembly from the extracted baseband signal (digital serial signal).
Transmission of the baseband signal starts with the slot boundary in every 625 .mu.s. A four-symbol (bit)-length preamble signal is first transmitted and then a 64 symbol (bit)-length syncword is subsequently transmitted. Finally, a variable-length payload is transmitted. The baseband receiving circuit of each bluetooth receiver monitors whether each 64 bit-length syncword exists in an input baseband signal. When the syncword addressed to the receiver itself has appeared, a payload thereafter is extracted into the receiving circuit and processed. Therefore, the baseband signal receiving circuit needs to detect a packet having a syncword addressed to the receiving circuit itself. For example, a syncword detector, which is referred to as a sliding correlator, is used when the syncword detector detects a packet which is addressed to the detector itself.
A sliding correlator is a circuit which acquires a correlation between a signal obtained by sampling an input baseband signal by a clock having a frequency (N MHz) equivalent to N times symbol frequency (1 MHz) of the input baseband signal and a 64 bit-length syncword. The sliding correlator stores the input baseband signal in a (63*N+1) bit-length shift-register and determines whether the syncword addressed to the receiver itself is transmitted or not by matching a 64 bit-length signal, which is extracted in every N bits starting with the top of the shift-register to a predetermined 64 bit-length syncword.
This sliding correlator has the advantage of being able to check stably whether the syncword has been detected within one symbol period even if the baseband signal includes noise. For example, a conventional baseband signal receiving circuit finally decides that the syncword is detected when this detected syncword period exceeds a threshold L (where L is greater than 0 and less than or equal to N), which is a continuous cycle number of the sampling frequency.
There is a possibility that a conventional baseband signal receiving circuit is unable to detect the syncword accurately when the syncword is detected intermittently within one symbol period due to a large amount of noise in a baseband signal. For this abovementioned reason, there has been a demand for a baseband signal detecting circuit which is able to regenerate symbols stably and accurately and a word detecting circuit which is able to detect word phases in the baseband signal, under the conditions described. Furthermore, there has been a demand for a syncword detecting circuit and a baseband signal receiving circuit that checks the history of previous N cycles of syncwords for stable syncword detecting and determines whether final syncword detection is successful or not by monitoring whether a detected number of matched bits in the history surpasses a threshold M (where M is greater than 0 and less than or equal to N).
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved a syncword detecting circuit and a baseband signal receiving circuit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.